Method of washing solid grain

ABSTRACT

In the washing process of the invention, the solid particles in a high-concentration zone, which is formed in a washing tank by a gravitational sedimentation of solid particles, are continuously washed by a counter-current contact with upward flow of a washing liquid which is fed from the bottom portion of the washing tank. With this process, the impurities in the solid particles are sufficiently removed by a simple apparatus. Since the used washing liquid can be recycled as the disperse medium for feeding the solid particles and as the washing liquid, the amount of used washing liquid to be discharged as the waste from the system is reduced.

TECHNICAL FIELD

The present invention relates to a process for washing solid particles,and more particularly to a process for efficiently washing solidparticles with a reduced amount of a washing liquid.

BACKGROUND ART

The washing of solid particles with a washing liquid has been frequentlycarried out in the production of organic and inorganic chemicalproducts. Recently, soils contaminated with harmful substances such asdioxin are washed with a washing liquid such as water for regeneration.

The washing of solid particles basically includes a step fortransferring impurities in the solid particles into a washing liquid anda step for separating the solid particles from the washing liquid. Inthe former stage, the impurities are removed from the solid particles bydissolution into the washing liquid or by dispersion into the washingliquid after divided into finer particles. A tank equipped with astirrer has been frequently used to enhance the removing efficiency andto increase the transferring speed of the impurities into the washingliquid. The impurities can be almost completely transferred into thewashing liquid by modifying the structure of the washing tank andcontrolling the residence time of the solid particles therein.

In the latter stage, the solid particles are separated by discarding thesupernatant after allowing a slurry to stand or by a solid-liquidseparation method such as filtration and centrifugal precipitation.However, a some amount of the washing liquid is generally retained inthe solid particles separated by these separation methods. The washingliquid itself retained in the solid particles is removed by drying, butimpurities in the washing liquid remains in the solid particles withoutbeing evaporated to result in an insufficient removal of the impurities.

Therefore, to sufficiently remove the impurities by washing the solidparticles, it is required to reduce the amount of the washing liquidthat accompanies the solid particles during the separation procedure. Toenhance the effect of washing the solid particles, there has been used aseparator in which a washing liquid containing impurities is removed bysprinkling a fresh washing liquid on the separated particles in theseparator. However, such a separator involves problems that thestructure is complicated and a sufficient washing effect is not obtainedwhen the size of solid particles are small. Another approach forenhancing the effect of washing the solid particles is a washing methodusing a combination of a number of washing tanks and separators. Sincethe centrifugal separators and rotary filter separators which arefrequently used in industrial processes are expensive, the method usinga number of these apparatuses increases installation costs. In addition,there is proposed a method of sufficiently washing solid particles usinga number of liquid cyclones (JP 5-140044 A). The cyclone itself is aninexpensive separator having a simple structure. However, a number ofpumps are required to recycle the washing liquid, this making theoverall system complicated. Therefore, the proposed method is notnecessarily inexpensive. Further, the proposed method is not applicableto solid particles that are easily crushed because the particles arecrushed in pumps and cyclones. Therefore, it has been demanded todevelop a method of sufficiently washing solid particles by using anapparatus with simpler structure.

Another problem to be solved upon washing solid particles is to reducethe amount of a used washing liquid to be discharged as the waste. Inthe washing of crystals for the production of chemical products and thewashing of contaminated soil as described above, the direct discharge ofthe used washing liquid causes environmental pollution. To avoid thisproblem, the used washing liquid should be discharged after decomposingthe impurities or making the impurities harmless by chemical orbiochemical treatments. It is advantageous for the decomposition or thetreatment of making harmless that the amount of the waste liquid issmaller and the impurities are more concentrated therein, because thesize of apparatus can be reduced and the energy required can be saved.In case of the removal of harmful substances such as dioxin which mustbe removed to an extremely low concentration, the waste liquid isdifficult to be made harmless efficiently with low costs by theconventional methods, because the amount of the waste liquid is largeand the concentration of impurities in the waste liquid is low. Forexample, a waste water of the same amount as that of soil being washedmust be made harmless (Example 1 of JP 2001-113261 A), or a washingwater three times the amount of soil to be washed is required (Examplesof JP 2001-47027).

DISCLOSURE OF INVENTION

An object of the present invention is to provide a process capable ofsufficiently removing impurities in solid particles by washing the solidparticles with a washing liquid in a simple apparatus and capable ofreducing the amount of a used washing liquid to be discharged as thewaste.

As a result of extensive researches in view of solving the aboveproblems in the washing of solid particles, the inventors have foundthat the impurities in the solid particles are sufficiently removed andthe amount of a used washing liquid to be discharged is considerablyreduced by feeding the solid particles and the washing liquid into awashing tank to form a high-concentration zone of the solid particles inthe washing tank and bringing the solid particles into counter-currentcontact with an upward flow which is formed by a part of the washingliquid fed. The present invention has been accomplished on the basis ofthis finding.

Thus, the invention provides a process for continuously washing solidparticles comprising:

(1) feeding the solid particles into a washing tank from an upperportion thereof and allowing the solid particles to gravitationallysediment, thereby forming a high-concentration zone of the solidparticles in the washing tank;

(2) feeding a washing liquid into the washing tank from a bottom portionthereof so that a part of the washing liquid fed forms an upward flow;

(3) bringing the solid particles into counter-current contact with theupward flow of the washing liquid;

(4) discharging washed solid particles as a slurry together with a partof remainder of the washing liquid; and

(5) separating the washed solid particles from the slurry.

With the continuous washing method of solid particles of the invention,the impurities in the solid particles are sufficiently removed and theamount of a used washing liquid to be discharged as the waste isreduced. Therefore, the costs for treating the used washing liquid isreduced to provide an industrially quite advantageous washing method ofsolid particles. In addition, the mother liquor left after separatingthe washed solid particles from the slurry can be used as the dispersemedium for the solid particles to be fed into the washing tank from itsupper portion or as the washing liquid to be fed into the washing tankfrom its bottom portion.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic illustration showing a process for washing solidparticles according to the present invention.

FIG. 2 is a schematic illustration showing a washing process in whichsolid particles are fed into a washing tank after mixed with a dispersemedium in a slurry preparation tank and a mother liquor separated in asolid-liquid separator is recycled as a washing liquid.

FIG. 3 is a schematic illustration showing a washing process in whichsolid particles are fed into a washing tank after mixed with a dispersemedium in a slurry preparation tank and a mother liquor separated in asolid-liquid separator is recycled as a disperse medium for preparing aslurry.

FIG. 4 is a schematic illustration showing a process for washing solidparticles employed in Comparative Examples 1 and 2, in which acombination of a common washing tank and solid-liquid separator is used.

FIG. 5 is a schematic illustration showing a stirring blade used inexamples, in which the upper is a top plan view, the lower is a sideview, and D is the inner diameter of washing tank.

FIG. 6 is a schematic illustration showing another stirring blade usedin examples, in which the upper is a top plan view, the lower is a sideview, and D is the inner diameter of washing tank.

FIG. 7 is a schematic illustration showing a washing apparatus used inExamples 8 and 9.

FIG. 8 is a schematic illustration showing a stirring blade used inExamples 8 and 9, in which the upper is a top plan view and the lower isa side view.

BEST MODE FOR CARRYING OUT THE INVENTION

The washing operation of the solid particles referred to herein includesoperations generally employed to reduce the content of impurities in thesolid particles by using a washing liquid, such as an operation ofremoving impurities attached to the solid particle surface by dissolvingthe impurities in a washing liquid, an operation of removing impuritiesinside the solid particles by extracting the impurities with a washingliquid, and an operation of obtaining washed solid particles byseparating a solvent containing impurities from a slurry produced by thechemical reaction in the solvent.

The shape and structure of the washing tank used in the presentinvention is not particularly limited. For example, the vertical washingtanks 2, 34 shown in FIGS. 1-3 and 7 may be preferably used.

The continuous washing of solid particle of the invention will beroughly described below. The solid particles are fed into the washingtank as they are (FIG. 1) or in a slurry form (FIGS. 2, 3 and 7) from afeed port at an upper portion of the washing tank. The solid particlesfed are allowed to gravitationally sediment in the washing tank to forma high-concentration zone of solid particles. The washing liquid is fedinto the washing tank from its bottom portion. A part of the washingliquid fed forms an upward flow which is then brought intocounter-current contact with the solid particles in thehigh-concentration zone to wash the solid particles. The washed solidparticles are discharged from the bottom portion of the washing tank asa slurry together with a part of the remainder of the washing liquid.After the counter-current contact, the upward flow of the washing liquidfurther rises to drain from a used washing liquid outlet at the upperportion of the washing tank. In case of feeding the solid particles in aslurry form together with a disperse medium, a major part of thedisperse medium in the slurry drains from the used washing liquid outlettogether with the upward flow of the washing liquid. The washing tank isgenerally operated at 0 to 230° C. under 0 to 10 MPaG (gauge pressure).

To reduce the amount of solid particles draining from the used washingliquid outlet, it is preferred to dispose the used washing liquid outletat a portion higher than the position of the solid particle/slurry feedport. In the washing tank shown in FIG. 1 for directly feeding the solidparticles as they are, the lower end of the solid particle feed port ispreferably positioned below the used washing liquid outlet. With such aconstruction described above, the solid particles are washed whilepreventing the impurity-rich liquid at the upper portion of the washingtank from flowing down to mix with the liquid at the bottom portion.

In the process of the present invention, it is important to form ahigh-concentration zone of solid particles in the washing tank. Thehigh-concentration zone may be formed by controlling the dischargeamount of the slurry from the bottom portion of the washing tank. If theconcentration of the solid particles in the high-concentration zone istoo low, the solid particles and the liquid therein undergo a vigorousconvection mixing to reduce the effect of removing impurities. If theconcentration of solid particles is too high, the blocking of solidparticles and the clogging of the slurry discharge port come to easilyoccur to make a stable operation difficult. The concentration of solidparticles in the high-concentration zone is preferably 15 to 50% byvolume.

The concentration of solid particles in the high-concentration zone maybe controlled by changing the feeding rates of solid particles and thewashing liquid. To form a stable high-concentration zone over a wideranges of the feeding rates, it is preferred to use a washing tankequipped with a stirrer. To prevent the flow of solid particles in thevertical direction, preferably used is a stirrer comprising a centralshaft and a plurality of stirring blades which form horizontal circularflows by rotation and are fitted to the central shaft along its verticaldirection. The shapes of the stirring blades capable of forming circularflow are illustrated in FIGS. 5, 6 and 8. The diameter of the stirringblade is preferably 0.5 to 0.99 time the inner diameter of the washingtank. The rotation speed of the stirring blade is preferably 0.2 to 5m/s in terms of a peripheral speed of its tip end. If the rotation speedis too low, the effect of preventing the vertical convection flow ofsolid particles is lowered. If the rotation speed is too high, anexcessive mixing is caused. In both cases, the effect of removingimpurities is lowered. To prevent the deposition of solid particles atthe bottom and the clogging of the slurry discharge port, a stirringblade having a different shape from the other blades, for example, aslant paddle blade and a turbine blade, may be used as the lowermoststirring blade which is disposed near the bottom of the washing tank.

To enhance the washing effect, it is preferred to increase the height ofthe high-concentration zone by increasing the height of the washing tankand to increase the number of stirring blades. The number of stirringblades to be generally used is 1 to 30. The stirring blades are arrangedat interspaces of a given level or more, preferably 0.1 to 2 times andmore preferably 0.2 to 1.5 times the diameter of the washing tank. Theheight of the high-concentration zone (from the bottom of washing tankto its upper surface) is preferably 0.5 to 0.95 time the height of theused washing liquid outlet form the bottom of washing tank. In case ofusing a washing tank equipped with a central shaft having a plurality ofstirring blades, the height of the high-concentration zone is preferably1.03 to 1.5 times the uppermost blade from the bottom of washing tank.

The flow rate of the upward flow of washing liquid is one part by weightor less and preferably 0.5 part by weight or less per one part by weightof the solid particles being treated. The smaller the amount of theupward flow of washing liquid is, the more preferred, because someportion of the upward flow are discharged as the waste out of thesystem. However, since an excessively low flow rate reduces the effectof removing impurities, the flow rate is preferably 0.01 part by weightor more per one part by weight of the solid particles being treated. Theflow rate (upward linear velocity) exceeding zero is enough to form theupward flow of washing liquid, and the upper limit is preferably about3.3 m/h.

The slurry discharged from the washing tank is introduced into thesolid-liquid separator. In case of operating the washing tank underhigh-temperature and high-pressure conditions, a storage tank ispreferably disposed before the solid-liquid separator to reduce thetemperature and pressure of slurry to suitable levels for treatments inthe solid-liquid separator. The storage tank is not required if thesolid-liquid separator is operable under high-temperature andhigh-pressure conditions. Examples of the solid-liquid separator includea centrifugal sediment separator, a centrifugal filter separator, avacuum filter and a pressure filter, although not limited thereto. Sincethe slurry is continuously discharged from the washing tank, thesolid-liquid separator to be used is preferably of a type capable ofcontinuously receiving the slurry and continuously discharging aseparated cake and a mother liquor. The mother liquor left afterseparating the solid particles from the slurry may be recycled as thewashing liquid for the solid particles. If the disperse medium is thesame as the washing liquid, the mother liquor may also be recycled asthe disperse medium.

Next, the solid particles, the washing liquid and the disperse mediumsuitably usable in the present invention will be explained.

The solid particles are allowed to gravitationally sediment in thewashing process of the invention. If the size of the solid particles istoo small, the sedimentation velocity is low to result in the failure intreating a sufficient amount of solid particles. On the contrary, if thesize is too large, the sedimentation velocity of the solid particlesbecomes too high to result in the failure in attaining a sufficientwashing effect. Therefore, the size of solid particles is preferably0.01 to 5 mm and more preferably 0.02 to 2 mm in terms of a mediandiameter on volume basis. If the solid particles to be washed have aparticle size distribution, fine particles escape in some cases from theused washing liquid outlet together with the upward flow of the washingliquid. Particles having a diameter of 0.005 mm or less usually escapefrom the used washing liquid outlet together with the upward flow of thewashing liquid without sedimenting, although depending upon theproperties of the washing liquid and the disperse medium for slurry. Ifthe escape of fine particles should be prevented, the lower limit of theparticle size distribution is preferably 0.005 mm or more.

The content of impurities tends to increase with decreasing particlesize in some cases. This may be because that the finer the particles,the larger the surface area becomes to let the impurities adhere orattach more easily, or that the finer the particles, the larger theamount of liquid retained in the solid particles after solid-liquidseparation. Therefore, if the fine particles containing the impuritiesin a relatively high content escape, the content of impurities of thesolid particles discharged from the bottom portion of the washing tankis reduced to enhance the washing effect. Therefore, if the amount offine particles escaping together with the used washing liquid is withinthe tolerable range, the escape thereof creates rather beneficialresults.

Examples of the solid particles to be washed include aromaticpolycarboxylic acids that are aromatic hydrocarbons having one or morearomatic rings such as benzene, naphthalene and biphenyl having theiraromatic rings substituted by two or more carboxyl groups.

As the benzene polycarboxylic acids, preferred are isophthalic acid,etc. except for terephthalic acid. Examples of the naphthalenepolycarboxylic acids include naphthalene dicarboxylic acids, naphthalenetricarboxylic acids and naphthalene tetracarboxylic acids, withnaphthalene dicarboxylic acids being preferred because of their utilityas raw materials for polyesters, urethanes and liquid crystal polymers,and 2,6-naphthalene dicarboxylic acid being more preferred. Examples ofthe biphenyl polycarboxylic acids include biphenyl dicarboxylic acids,biphenyl tricarboxylic acids and biphenyl tetracarboxylic acids, withbiphenyl dicarboxylic acids being preferred because of their utility asraw materials for polyesters, polyamides and liquid crystal polymers,and 4,4′-biphenyl dicarboxylic acid being more preferred.

Taking the dissolving power to the solid particles and the impurities tobe removed, the specific gravity and the viscosity into consideration,the washing liquid is selected from water, aliphatic carboxylic acidssuch as acetic acid, aliphatic hydrocarbons, aromatic hydrocarbons,esters such as carboxylic esters, alcohols, ketones, etc. Preferably,the washing liquid has a sufficient dissolving power to the impuritiesto be removed from the solid particles, but has a dissolving power notso high to the solid particles to be washed. More specifically, it ismore preferred that the washing liquid dissolves the impuritiescompletely at the operating temperature of washing tank, and that thedissolving power to the solid particles to be washed is less than 10 gper 100 g of the washing liquid.

To allow the solid particles to gravitationally sediment, the specificgravity of the washing liquid should be less than the true specificgravity of the solid particles. The sedimentation velocity of the solidparticles varies depending upon the specific gravity difference betweenthe solid particles and the washing liquid and the viscosity of thewashing liquid. Since a sedimentation velocity which is too high or toolow brings about unfavorable results as mentioned above, the solidparticles and the washing liquid are preferably combined so as to attainan appropriate sedimentation velocity. Specifically, preferred is awashing liquid allowing a terminal sedimentation velocity of preferably0.0005 to 0.5 m/s, more preferably 0.001 to 0.15 m/s at the averageparticle size of solid particles.

The disperse medium used for feeding the solid particles in slurry formmay be the same as or different from the washing liquid and may beselected like the washing liquid. If different from the washing liquid,it is preferred that the disperse medium and the washing liquid aremutually dissolved at any ratio to form a uniform solution.

To enhance the washing effect, additives such as surfactants may beadded to the washing liquid or the disperse medium for slurry.

The apparatus systems for practicing the washing process of the presentinvention are illustrated in FIGS. 1-3 and 7. FIG. 1 shows a washingprocess in which solid particles 11 are directly fed into a washing tank2. FIGS. 2 and 3 show a process in which the solid particles 11 aremixed with a disperse medium 12 in a slurry preparation tank 1 and thenfed into the washing tank 2. This process is suitable when the washingtank is operated under high-temperature and high-pressure conditions toenhance the washing effect and when the solid particles in a slurryobtained by a chemical reaction in a solvent are washed. In FIG. 2, amother liquor 18 separated in a solid-liquid separator is recycled as awashing liquid 14, and in FIG. 3, the separated mother liquor 18 isrecycled as the disperse medium 12 for slurry. In the process shown inFIG. 7, the slurry is fed into a washing tank 34 from a slurrypreparation tank 31. In the attached drawings, a means for transportingliquid such as a pump and a heating or cooling device such as a heatexchanger are omitted. In FIGS. 1-4, like reference numerals indicatelike parts.

Referring to FIG. 2, the invention is explained in more detail below.The solid particles 11 are fed into the slurry preparation tank 1 andmixed with the disperse medium 12. In the process for washing the solidparticles in a slurry obtained by a chemical reaction in a solvent, thereference numerals 11, 12 and 1 respectively correspond to a rawmaterial for the solid particles, a reaction solvent and reactor.

The structure of the slurry preparation tank is not particularly limitedas long as its size is sufficient to prepare a slurry by mixing thesolid particles and the disperse medium. To intimately mix the solidparticles and the disperse medium and prevent the deposition oraggregation of the solid particles, a stirrer may be provided in theslurry preparation tank.

The slurry from the preparation tank 1 is fed to the washing tank 2through a line 13. The solid particles fed to the washing tank 2 areallowed to gravitationally sediment while forming a high-concentrationzone of the solid particles in the washing tank, and finally dischargedas a slurry with a washing liquid 14 from a bottom portion of thewashing tank through a line 15. A major part of the disperse medium 12in the slurry drains through a line 21 from a used washing liquid outletwhich is located above the slurry feed port. The washing liquid 14 isfed from the bottom portion of the washing tank 2. A part of the washingliquid 14 rises as an upward flow in the washing tank. The upward flowof the washing liquid is brought into a counter-current contact with thesolid particles 11 and then drains from the used washing liquid outlet.In this manner described above, the solid particles are washed whilepreventing the impurity-rich liquid at the upper portion of the washingtank from coming down to mix with the liquid at the bottom portion.

The slurry discharged from the bottom portion of the washing tank is fedinto a solid-liquid separator 4 through a line 15, a slurry storage 3and line 16 and separated into a cake 17 and a mother liquor 18. Byremoving the washing liquid retained in the separated cake 17, thewashed solid particles are obtained as the final product. A part of themother liquor 18 from the solid-liquid separator may be recycled as thewashing liquid 14 through a line 19, or as the disperse medium 12 forpreparing the slurry as shown in FIG. 3. The mother liquor which is notrecycled is discharged from the system through a line 20. As the amountof mother liquor recycled increases, the amount of mother liquordischarged as the waste from the system is preferably reduced. In theprocess of the invention, substantially the complete amount of theseparated mother liquor can be recycled.

A part of the used washing liquid 21 draining from the used washingliquid outlet of the washing tank 2 may be recycled through a line 23 asthe disperse medium 12 for preparing the slurry. As the recycled amountincreases, the impurities are concentrated more in the used washingliquid 21 to facilitate the treatment for making the impuritiesharmless. In addition, the amount of used washing liquid 22 to bedischarged from the system is reduced. If the washing liquid isexpensive and noxious to the environment, the impurities in the usedwashing liquid should be separated or decomposed for regeneration orreuse of the washing liquid without discharging the used washing liquidfrom the system. The used washing liquid is regenerated, for example, bydistillation. Therefore, it is quite advantageous that the amount ofused washing liquid is small, because the energy required forregeneration can be saved and the size of regeneration facilities can bereduced.

The present invention will be described in more detail by reference tothe examples, but it should noted that the examples are not intended tolimit the invention thereto.

EXAMPLE 1

Using the apparatus shown in FIG. 1, the experiment for removingimpurities attached to the surface of solid particles was conducted. Asthe solid particles, quartz sand (Ube Sand #7, average particlesize=0.10 mm, true specific gravity=2.6) available from Ube Sand KogyoCo., Ltd. was used. To determine the effect of removing impurities, thequartz sand was immersed in an aqueous sodium chloride solution,subjected to solid-liquid separation, and then dried to obtain raw solidparticles, which were fed to the washing tank. The sodium ion content ofthe raw solid particles was 830 ppm by weight. Water was used as thewashing liquid.

The washing tank comprised a cylindrical portion having an innerdiameter of 300 mm and a conical bottom portion, and had a slurrydischarge port at its lowermost portion. The cylindrical portion was2,000 mm long and had a feed port for solid particles at its topsurface. A used washing liquid outlet was disposed 200 mm below the topsurface of the washing tank. The lower end of the nozzle of the feedport for solid particles was located 400 mm below the top surface of thewashing tank. The washing tank was fitted with a central shaft havingnine stirring blades (blade diameter=270 mm) shown in FIG. 5 atinterspaces of 150 mm and one flat paddle blade as the lowermost bladewhich had a shape along the bottom portion at the lowermost position.

The slurry discharged from the bottom portion of the washing tank wasfed to the solid-liquid separator by a pump (not shown). Thesolid-liquid separator used was a centrifugal precipitation type. Theseparated solid particles were dried and then measured for attachedsodium ions.

After filling the washing tank with water, the raw solid particles andthe washing water were fed at respective rates of 100 parts by weight/hand 20 parts by weight/h while rotating the stirrer at 60 rpm. Ahigh-concentration zone of solid particles was formed in the washingtank without discharging the slurry from the bottom portion. When theupper surface of the high-concentration zone reached 200 mm above theuppermost stirring blade, the discharge of the slurry from the bottomportion and the feeding of the slurry to the separator were started. Themother liquor obtained in the separator was completely recycled to thewashing tank as the washing liquid through the recycling line.Thereafter, the washing tank was continuously operated while controllingthe amount of the slurry discharged from the bottom portion so as tomaintain the upper surface of the high-concentration zone at constantlevel, and simultaneously, controlling the feeding amount of washingwater so as to allow the used washing liquid to drain from the usedwashing liquid outlet in a rate of about 10 parts by weight per onehour. During the operation, the concentration of solid particles in thehigh-concentration zone was 25 to 26% by volume.

The separated solid particles were dried and measured for the watercontent and the residual sodium ion concentration. The water content was5 to 6% by weight and the sodium ion concentration was 5.2 to 6.1 ppm onthe washed solid particles sampled after reaching a stable operation anddried. The removal of sodium ions based on the raw solid particles was99.27 to 99.37%.

Comparative Example 1

Using an apparatus for washing solid particles comprising a combinationof a common washing tank and a common solid-liquid separator as shown inFIG. 4, an experiment for evaluating the effect of removing impuritieswas conducted. The washing tank was equipped with a stirrer having slantpaddle blades. The solid-liquid separator was the same type as used inExample 1. The same solid particles as used in Example 1 and a washingwater were fed to the washing tank at respective rates of 100 parts byweight/h and 250 parts by weight/h. The discharged slurry was fed to theseparator by a pump. The separated mother liquor (about 240 parts byweight) was not reused and completely discharged from the system.

The separated solid particles were analyzed in the same manner as inExample 1. The water content was 5 to 6% by weight, the sodium ionconcentration was 17 to 20 ppm, and the removal of sodium ions was 97.6to 97.9%.

The amount of the used washing liquid discharged from the system wasvery large and the removal of impurities was low as compared to those ofExample 1.

Comparative Example 2

The procedure of Comparative Example 1 was repeated except that thewashing liquid was fed at a rate of 15 to 16 parts by weight/h and apart of the separated mother liquor was discharged from the system at arate of 10 parts by weight/h while recycling the remainder to thewashing tank.

The water content was 5 to 6% by weight, the sodium ion concentrationwas 280 to 320 ppm, and the removal of sodium ions was 33 to 38%.

Although the amount of the used washing liquid discharged from thesystem was nearly the same as in Example 1, the removal of impuritieswas considerably poor.

EXAMPLE 2

The procedure of Example 1 was repeated except that the feeding amountof the washing water was controlled so as to allow the used washingliquid to drain at a rate of about 30 parts by weight/h.

The sodium ion concentration was 0.58 to 0.63 ppm and the removal ofsodium ions was 99.92 to 99.93%.

EXAMPLE 3

The procedure of Example 1 was repeated except that a part of the motherliquor separated in the separator was discharged from the system at arate of 10 parts by weight/h while recycling the remainder as thewashing water.

The sodium ion concentration was 1.8 to 2.1 ppm and the removal ofsodium ions was 99.75 to 99.78%.

EXAMPLE 4

The procedure of Example 1 was repeated except for changing the numberof stirring blades to five and the interspaces to 300 mm. The removal ofsodium ions was 98.2 to 98.3%.

EXAMPLE 5

The procedure of Example 1 was repeated except for changing the rotationspeed of stirring blade to 150 rpm (peripheral speed of the tip end ofblade=2.1 m/s). The removal of sodium ions was 97.3 to 97.5%.

EXAMPLE 6

The procedure of Example 1 was repeated except for using the stirringblades shown in FIG. 6. The removal of sodium ions was 97.2 to 97.8%.

Comparative Example 3

The procedure of Example 1 was repeated except for changing the feedingamount of solid particles to 250 parts by weight/h and the drainingamount of used washing liquid to 30 parts by weight/h. During theoperation, the concentration of solid particles in thehigh-concentration zone was about 14% by volume.

The water content was 5 to 7%, the sodium ion concentration was 150 to170 ppm, and the removal of sodium ions was 79 to 82%.

Comparative Example 4

The procedure of Example 1 was repeated except for changing the rotationspeed of the stirrer to 10 rpm (peripheral speed of the tip end ofblade=0.14 m/s). The removal of sodium ions of 76 to 80%.

EXAMPLE 7

The procedure of Example 1 was repeated except for changing the quartzsand to granular alumina (average particle size=0.20 mm; specificgravity=2.0). The granular alumina fed had a sodium ion concentration of970 ppm.

The water content was about 6%, the sodium ion concentration was 8.3 to8.8 ppm, and the removal of sodium ions was 99.09 to 99.14%.

EXAMPLE 8

Using the apparatus shown in FIG. 7, an acetic acid solvent slurry(crude slurry) of crude isophthalic acid crystals obtained by theliquid-phase oxidation of m-xylene was washed with water. The crudeslurry was produced in industrial scale by oxidizing m-xylene at 200° C.in a water-containing acetic acid solvent in the presence of anoxidation catalyst comprising cobalt, manganese and a bromine compoundwhile blowing air into the solvent. The concentration of isophthalicacid crystals in the crude slurry was 30% by weight, and the motherliquor after removing the crystalline component consisted of 86% byweight of acetic acid and 14% by weight of water.

Referring to FIG. 7, the crude slurry in a preparation tank 31 was fedto an upper portion of a washing tank 34 through a line 33 by driving apump 32. The washing tank 34 was constructed by a titanium cylinderhaving an inner diameter D of 36 mm and equipped with a stirring shaft36 connected to a motor 35. The stirring shaft 36 was provided withfifteen stirring blades 37 at 50-mm interspaces at its portion below afeed port for the crude slurry. The stirring blades shown in FIG. 8 wereused. The diameter d of the stirring blade was 32 mm, being about 0.9time the inner diameter D. An outlet pipe 39 for used washing liquid wasdisposed at the top portion of the washing tank 34. At the bottomportion of the washing tank 34, a feeding pipe 40 for the washing liquidand a discharging pipe 41 for the washed slurry were disposed. Thewashing liquid was fed to the washing tank 34 by means of a pump 42. Inthe lines 33, 40 and 41, flow meters and flow control valves (not shown)were provided. In a line 39, a valve (not shown) for controlling theinner pressure of the washing tank was provided.

First, the washing tank was filled with water of 90° C. by driving thepump 42. When water began to overflow from the outlet pipe 39 for usedwashing liquid, the feeding amount of water was controlled so as toadjust the upward linear velocity of water flow to 0.5 m/h. Then, theshaft 36 and the stirring blades 37 were rotated at 120 rpm by drivingthe motor 35. The peripheral speed of the tip end of stirring blade was0.20 m/s. Next, the pump 32 was operated to feed the crude slurry of160° C. from a feeding nozzle 38 through the line 33 at a flow rate of8.3 kg/h.

When it was confirmed by the monitor using a powder level detector thatthe height of the high-concentration zone reached 50 mm above theuppermost stirring blade, the feeding amount of the washing water wasincreased and the discharge of the slurry from the bottom portion ofwashing tank was started. The discharged slurry was stored in a storagetank 43. The amount of the slurry being discharged was controlled so asto maintain the height of the high-concentration zone at the intendedlevel, and simultaneously, the amount of the washing water being fed wascontrolled so as to maintain the upward linear velocity of water flow atthe intended level (0.5 m/h). The operation was continued for 4 h afterthe system was stabilized, and a sample was taken out of the dischargedslurry. The sample was subjected to solid-liquid separation and dried toobtain isophthalic acid crystals. The hue of the crystals expressed byOD₃₄₀ was 0.71.

OD₃₄₀ is the absorbance at 340 nm and measured by a spectrophotometer ona filtrate in 50-mm quartz cell, which filtrate was prepared bydissolving 5.0 g of isophthalic acid crystals in 30 ml of 3N ammoniawater and filtering through a 5-μm membrane filter.

Separately, an acetic acid solvent slurry of isophthalic acid producedin industrial scale was subjected to solid-liquid separation using arotary vacuum filter (RVF) and then dried to obtain crude isophthalicacid crystals. OD₃₄₀ was 2.42.

EXAMPLE 9

Using the apparatus shown in FIG. 7, an acetic acid solvent slurry(crude slurry) of crude 2,6-naphthalenedicarboxylic acid crystalsobtained by the liquid-phase oxidation of 2,6-dimethylnaphthalene waswashed with water. The crude slurry was produced in pilot apparatus byoxidizing 2,6-dimethylnaphthalene at 200° C. in a water-containingacetic acid solvent in the presence of an oxidation catalyst comprisingcobalt, manganese and a bromine compound while blowing air into thesolvent. The concentration of 2,6-naphthalenedicarboxylic acid crystalsin the crude slurry was 28% by weight, and the mother liquor afterremoving the crystalline component consisted of 88% by weight of aceticacid and 12% by weight of water.

The procedure of Example 8 was repeated except for feeding the crudeslurry of 190° C. at a rate of 50 g/h. The operation was continued for 4h after the system was stabilized, and a sample was taken out of thedischarged slurry. The sample was subjected to solid-liquid separationand dried to obtain 2,6-naphthalenedicarboxylic acid crystals. The hueof the crystals expressed by OD₄₀₀ was 0.78.

OD₄₀₀ is the absorbance at 400 nm and measured by a spectrophotometer ona filtrate in 10-mm quartz cell, which filtrate was prepared bydissolving 1.0 g of 2,6-naphthalenedicarboxylic acid crystals in 10 mlof 1N NaOH aqueous solution and filtering through a 5-μm membranefilter.

Separately, an acetic acid solvent slurry of 2,6-naphthalenedicarboxylicacid produced in industrial scale was subjected to solid-liquidseparation using a basket centrifugal separator and then dried to obtaincrude 2,6-naphthalenedicarboxylic acid crystals. OD₄₀₀ was 2.13.

INDUSTRIAL APPLICABILITY

The process of the present invention is applicable to various washingoperations such as an operation of removing impurities attached to thesolid particle surface by dissolving the impurities in a washing liquid,an operation of removing impurities inside the solid particles byextracting the impurities with a washing liquid, and an operation ofobtaining washed solid particles by separating a solvent containingimpurities from a slurry produced by the chemical reaction in thesolvent. Thus, the invention is industrially useful.

1. A process for continuously washing solid particles comprising: (1)feeding solid particles into a washing tank from an upper portionthereof and allowing the solid particles to gravitationally sediment,and forming a high-concentration zone of the solid particles in thewashing tank, wherein a concentration of the solid particles in thehigh-concentration zone is 15-50% by volume, wherein thehigh-concentration zone is stirred by a stirrer, the stirrer having adisc-form stirring blade, wherein a rotation speed of the stirrer iscontrolled to a speed in a range of 0.84 to 5 m/s, in terms of aperipheral speed of its tip end, and wherein the stirring is made so asto form circular flows in the high-concentration zone by the stirrer,the stirrer comprising a stirring shaft extending, in a verticaldirection and a plurality of stirring blades fitted to the stirringshaft along the vertical direction; (2) feeding a washing liquid intothe washing tank from a bottom portion of the washing tank so that apart of the washing liquid fed forms an upward flow; (3) bringing thesolid particles into counter-current contact with the upward flow of thewashing liquid to wash the solid particles; (4) discharging the washedsolid particles as a slurry together with a part of a remainder of thewashing liquid; and (5) separating the washed solid particles from theslurry, wherein a height of the high-concentration zone, from a bottomof the washing tank, is 0.5 to 0.95 times a height, from the bottom ofthe washing tank, of an outlet of the washing liquid from the washingtank, after the washing liquid has been used to wash the solidparticles.
 2. The process according to claim 1, wherein the solidparticles are fed to the washing tank as a slurry together with adisperse medium.
 3. The process according to claim 1, wherein a part ofa mother liquor left after separating the washed solid particles fromthe discharged slurry is recycled as the washing liquid.
 4. The processaccording to claim 1, wherein the solid particles are aromaticpolycarboxylic acid crystals.
 5. The process according to claim 1,wherein the washing tank has a length extending in the verticaldirection, the stirring being made to form horizontal circular flows inthe high-concentration zone.
 6. The process according to claim 1,wherein the washing liquid, after being used to wash the solidparticles, is discharged from the washing tank from a location of thewashing tank higher than a location at which the solid particles are fedto the washing tank.
 7. The process according to claim 1, wherein alowermost stirring blade, of the plurality of stirring blades, has ashape different from that of other stirring blades of the plurality ofstirring blades.
 8. The process according to claim 1, wherein a height,from the bottom of the washing tank, of the high-concentration zone is1.03 to 1.5 times a height, from the bottom of the washing tank, of anuppermost stirring blade of the plurality of stirring blades.
 9. Theprocess according to claim 1, wherein the rotation speed of the stirreris controlled to a speed in a range of 2.1 to 5 m/s, in terms of aperipheral speed of its tip end.
 10. The process according to claim 2,wherein a part of a mother liquor left after separating the washed solidparticles from the discharged slurry is recycled as the disperse medium.